The present invention relates to a monochromatic image forming optical system that is used for an observing system used for observations of the sun, and to a monochromatic filtering optical system employed in the monochromatic image forming optical system.
In the art of observations of the solar activity, light of very narrow wavelength range (about 0.05-0.1 nm) which can be regarded as monochromatic light, is used to form an image of the sun for observations. In general, the narrow wavelength range is defined by a bandwidth and its center wavelength of 656.3 nm (H.alpha.-line). To form such an image, i.e., the monochromatic image of the sun, a narrow bandpass interference filter is used as a monochromatic filter, which is located in an optical system of an astronomical telescope. The interference filter is located at a position adjacent to an image plane of an objective lens of the astronomical telescope such that the filter is orthogonal to the optical axis of the astronomical telescope.
Generally, the interference filter has an incident angle dependency. That is, the transmission characteristics (i.e., a relationship between transmittance and wavelength) varies as the incident angle varies. If an interference filter is designed for light which is incident on the filter at right angles, and if light is incident on the filter obliquely, an actual transmitting wavelength range shifts from a designed wavelength range. That is, when the incident angle of the incident light is different from the designed angle, the center wavelength of the wavelength range shifts from the designed one. Further, the narrower the designed bandwidth of an interference filter is, the smaller the allowable range of the incident angle is.
If the optical system consists of the objective lens and the monochromatic filter (i.e., the narrow bandpass interference filter) as described above, within axial rays incident on the monochromatic filter via the objective lens, the rays incident on the filter at a small ray height (i.e., the height with respect to the optical axis) has relatively small incident angles which can be regarded as approximately 0 degree; while, the rays incident on the filter at a large ray height are incident on the filter at relatively large incident angles.
Since the center wavelength of the wavelength range of the transmitting rays varies depending on the incident angle, the center wave length of the wavelength range of the light passed through the filter is shifted from the designed one, which lowers contrast of an image.
FIG. 7 shows a conventional optical system that includes an objective lens 1, a negative lens group 2, a positive lens group 3, and a monochromatic filter 4. In this optical system, by arranging the negative lens group 2 in the vicinity of an image formed by the objective lens 1, the focal length of the objective lens 1 is expanded, and the F-number of the entire system is increased. Further, the positive lens group 3 is arranged in the vicinity of the monochromatic filter 4 to constitute a telecentric system on an image plane 5 side.
With this construction, since the F-number is relatively large, the difference of the incident angles due to the difference of the incident ray heights onto the monochromatic filter 4 can be reduced. Further, the telecentric system reduces variation of the incident angle due to the difference of the image size. Therefore, the center wavelength of the transmission wavelength range will not be shifted in the entire area of the image plane, thereby providing high contrast monochromatic images.
The optical system of FIG. 7, however, has a problem such that the monochromatic filter 4 should be arranged at a position relatively close to the image plane 5 since the negative lens group 2 and the positive lens group 3 should be arranged sufficiently apart from each other to reduce coma and spherical aberration with respect to offaxial rays. Since the monochromatic filter 4 is close to the image plane 5, dust and/or dirt adhered onto the filter surface may easily cause shadow and/or flare to be formed in the image on the image plane 5. Further, as the distance between the lens groups 2 and 3 is made longer, the backfocus becomes shorter, which restricts an applicable type of an observing device such as an eyepiece or a camera.
Furthermore, since the focal length of the objective lens 1 is expanded with the negative lens group 2 in the optical system shown in FIG. 7, the size of the entire optical system is increased, and the size of the monochromatic image formed on the image plane is also increased as compared with the normal optical system consisting of the objective lens and the monochromatic filter. In order to capture the large-size image with a generally-used image capturing element such as a CCD (Charge Coupled Device), an additional optical system for reducing the size of the image is required.